This invention is concerned with techniques for processing information which is transmitted optically, such as in an optical communications system or in an optical computer.
The inherent parallelism of optics (i.e., a beam of light can carry different information on different portions of the light beam without interference) and the wide bandwidth which an optical system offers for communication make such a system ideal for applications such as real-time image processing, optical interconnection schemes, and associative processing. As a result, optics is emerging as an area of increasing importance in high-speed information processing. Real-time image processing is of particular interest in fields, such as robotics, which require the recognition and tracking of objects. A clear advantage to the optical approach for these applications is the capability of parallel processing, with its concomitant increase in processing speed over digital computing techniques. Additional applications for optical image processing include industrial quality assurance, optical logic gates, and the detection of motion in a scene. Real-time contrast reversal of an image is an important step in image algebra, which deals with mathematical operations such as the addition, subtraction, multiplication, and division of spatial images. See, e.g., Chiou, et al., Phase-Conjugate Interferometric Coherent Image Subtraction, U.S. Pat. No. 4,718,749; Yeh, Nonlinear Optical Matrix Manipulation, U.S. Pat. No. 4,767,197.
A large number of signal and image processing algorithms require the intensity inversion of a spatial image. The use of electronic digital processing for this operation is very slow because of its serial nature. By contrast, the optical approach, with its inherent parallelism, offers the potential for great improvement in the speed of such operations. Real-time contrast reversal can be achieved with several different approaches. In a manner very similar to image subtraction, for example, contrast reversal can be accomplished by subtracting an image from a uniformly illuminated picture containing no image information. In addition, real-time contrast reversal can also be attained by using image division (or intensity inversion) where the output intensity pattern is inversely proportional to the input intensity pattern. The latter technique takes advantage of the dependence of four-wave mixing efficiency on the intensity ratios between the beams in photorefractive media. Contrast reversal has also been demonstrated in Fe:LiNbO.sub.3 crystals using pump beam depletion in two-wave mixing. Recently, real-time contrast reversal has been achieved by using selective erasure of holographic gratings during two-wave mixing in a BaTiO.sub.3 photorefractive crystal.